Oxide superconducting wires and cables typically consist of many filaments of superconducting material within a metal matrix which separates the filaments from each other and from the local environment. The matrix is typically a non-superconducting metal. Silver and its alloys represent the matrix metals of choice because the silver is ductile, chemically benign with respect to the oxide superconductor material and relatively transparent to oxygen.
Recent advances in the development of oxide superconductors have demonstrated their utility in applications such as power transmission cables, fault current limiters, utility inductors, motors and generators. For optimal performance, however, many of these applications require matrix resistivities which are much higher than that of pure silver at use (i.e., cryogenic) temperatures. Pure silver has a resistivity at 80 K of about 0.2-0.5 .mu..OMEGA.-cm, and this value decreases by an order of magnitude as the temperature drops to 4 K. As the term is used herein, resistivity is defined as the bulk resistivity as determined by measuring the current flow in a wire and applying the formula ##EQU1##
where .rho. represents resistivity, V represents voltage measured over wire length x, A represents the cross-sectional area of the wire, and I represents current.
There are many technical difficulties associated with the manufacture of an oxide superconductor having a high resistivity sheath. For example, processing steps associated with the formation of the high resistivity sheath may not be compatible with the processing of the oxide superconductor. In particular, under high temperature conditions used to form oxide superconductor phases, many of the sheath components likely to impart high resistivity to the sheath react with and poison the oxide superconductor. In addition, metals which are chemically compatible with the oxide superconductor and the sheath metal typically are highly electrically conductive.
One approach to increasing matrix resistivity consists of the introduction of fine oxide particles into the metal matrix to form a dispersed oxide/matrix metal alloy ("oxide-dispersion strengthened" or ODS silver); however, this requires relatively large volume fractions of the oxide phase in order to sufficiently raise the bulk resistivity of the matrix. Such an approach is limited because an increase in the oxide content of the matrix metal increases its brittleness. Thus, only modest increases in resistivity, e.g., 1-2 .mu..OMEGA.-cm, are possible while maintaining a matrix with acceptable mechanical properties. In order not to crack in ordinary coiling and winding operations, the matrix should have a tensile fracture strain of at least 0.5%. Fracture strains of higher than 1% are preferred for practical handling of the superconducting composite. In addition, the oxide precipitates used in ODS silver often interact detrimentally with the oxide superconductor and tend to degrade the superconducting properties of the composite.
In another approach, a metal may be alloyed with the sheath metal prior to composite fabrication to raise the resistivity of the sheath. While many metals may be readily alloyed and incorporated into the metal sheath, this process requires that the solute metal be present during high temperature processing of the oxide superconductor. Unfortunately, known low-cost solutes which significantly increase resistivity typically poison the superconductor or themselves are subject to oxidation under these processing conditions.
Shiga et al. in U.S. Pat. No. 5,296,456 disclose alloying a variety of metals with the metal sheath covering the oxide superconductor to obtain high conductivity (low resistivity) and low conductivity (high resistivity) regions in the sheath. As is discussed in greater detail below, most of the metals disclosed by Shiga et al. are not very effective for increasing electrical resistivity. Further, many metals which are highly effective in raising the net resistivity of the matrix are not good candidates for alloying with the metal sheath because they tend to readily form second phases, e.g., intermetallic compounds, within the matrix metal. Intermetallics tend to embrittle the matrix, and do not raise net resistivity sufficiently.
Due to the limitations of prior art processes, a need remains for sheathed oxide superconducting composites which combine suitably high resistivity with good superconducting properties.